The development of highly efficient radio transmitters has been an objective of engineers since the 1930's. Recent developments in digital signal processing hardware have made possible the generation of multi-gigabit-per-second serial data streams. Such high speed data streams have been investigated for use as the driving waveform in high efficiency transmitters, in particular those employing switch mode Radio Frequency (RF) power amplifiers. An extension of this concept is the development of a transmitter which efficiently converts digital information directly to a radio frequency signal. Thus, there has been a push not only for highly efficient transmitters, but also for the ability to transition directly from a digital signal to a radio frequency signal.
An example of a Direct Digital to RF (hereinafter “DDRF”) transmitter is shown in FIG. 1. In such systems, I and Q information is converted through a digital signal processor (DSP) 102 into a format that is acceptable by a delta-sigma modulator 104. The delta-sigma modulator 104, usually a band-pass delta-sigma modulator, is used to create a binary drive waveform 106 (as shown by the binary output 106a, two-state, serial representation) which has desirable noise properties, such as a noise notch, in the vicinity of the carrier frequency as shown in the Delta-Sigma Output Spectrum 106b. This waveform is coupled to an efficient power amplifier configured in a basic H-Bridge 126 format as is used in digital audio or motor controller applications. The two-state drive causes current to flow in alternating directions through the load 128 presented to the H-Bridge Power Amplifier, thus developing an amplified version of the desired RF output 130.
The operation of the H-bridge Power Amplifier 126 proceeds as follows: The binary output 106a of the Delta-Sigma Modulator determines which Field Effect transistors (hereinafter “FETs”) are turned on or off. The binary signal is applied to the inputs of two inverting and two non-inverting driver stages. The driver outputs determine the gate-source voltage applied to each of the FETs, and are so configured as to permit only one diagonally opposite pair of FETs to be turned on at any given time. As an example, if a plus-one is transmitted to the H-bridge Power Amplifier 126, the FET1 132 and the FET4 136 are turned on. As a result, current flows from FET1 132 through the load 128 to FET4 136. When a minus-one (−1) is transmitted to the H-bridge Power Amplifier 126, the FET2 134 and the FET3 138 are turned on. This causes current to flow from FET2 134 through the load 128 to FET3 138. As can be seen, the current through the load 128 changes direction as the different pairs of FETs, 132 and 136 or 134 and 138, are turned on and off. This operation provides an alternating current to the load and delivers power to the load.
The signaling scheme described is a “+1” “−1” waveform. The rapid rise and fall times of such “square-edged” waveforms allow operation of the Power Amplifier (hereinafter “PA”) devices (bipolar or FET) as idealized switches. Such an operation minimizes the power dissipation within the PA devices, leading to significant drain (or collector) efficiency improvement over that attained using traditional “sinusoidal” driving functions. The bandwidth of the “+1” “−1” waveform greatly exceeds that of the desired transmission channel bandwidth, extending from DC to at least 4 times the carrier frequency. Because of this bandwidth, a suitable band-pass filter is placed at the PA output to reject the unwanted off-channel energy and prevent such components from contributing to the “DC input” power required by the PA. In this regard, the H-bridge 126 has the unique property of being able to accept a broadband drive signal yet discriminate against the DC input components corresponding to those spectral components which exist outside of the desired channel bandwidth. This acceptance of a broadband signal and discrimination of the DC input components is accomplished by the four device PA topology in conjunction with band pass filtering placed in series with the load. The DC component dissipated by the PA is restricted to that necessary to support the portion of the spectrum which is associated with the on-channel signal.
The binary output 106a of delta-sigma modulator is an undesirable driving waveform for RF applications. The two signal states represented by the plus-one (+1) minus-one (−1) waveform transfers a constant level of drive power to the PA, regardless of the signal amplitude delivered to the load 128. As many modulation systems such as CDMA and OFDM statistically spend much time at low power levels, the constant drive of the binary waveform implies the stage gain of the power amplifier will be low. In turn, the low gain requires significant drive power to be applied to the PA, resulting in low efficiency operation of the overall system. The need is therefore established for a direct digital to RF transmitter wherein the gain of the PA is maintained at a high level to promote high efficiency operation.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.